This invention relates in general to machines for slitting sheet metal and, more particularly, a slitting machine having knives that are positioned quickly and with considerable precision, to a machine that affords easy and quick access to the knives, and to a machine in which the knives minimize distortions at the slits formed by the machine.
Steel mills furnish steel sheet in coils which rarely correspond in width to any of the multitude of products that are produced from the sheet. To facilitate the manufacture of such products, the steel sheet of a coil is withdrawn from the coil and slit longitudinally to widths suitable for the products. Often the sides are trimmed to provide undistorted side edges, and this produces more strips, although quite narrow. Slitting machines exist to slit the metal sheet. Metal strips derived from a slitting machine are then passed through a shear where they are cut into segments of appropriate length or else they are rewound into coils for subsequent processing or use.
The typical slitting machine has a frame in which upper and lower arbors rotate. The arbors carry knives, which in essence are disks having cylindrical peripheral surfaces that approach each other at a nip, yet are offset axially so that the side face of the upper disk lies slightly beyond the opposite side face of the lower disk. Moreover, at the nip the spacing between the cylindrical peripheral surfaces of the offset knives is less than the thickness of the sheet. Thus, as the steel sheet passes into and through the nip formed by a pair of knives, it undergoes a fracture which creates the slit.
The knives need to be positioned on their respective arbors with a good measure of precision, not only to ensure that the metal strips emerging from the machine have the correct width, but also to ensure that clean fractures occur within the sheet. A packed arbor utilizes spacers and shims to position the knives on it. Sometimes a computer selects the spacers and shims, but even so, setting up two packed arbors can consume considerable time. Moreover, tolerance build-up in the numerous spacers and shims can produce inaccurate dimensions in the strips. And of course the possibility of operator error exists. A programmable or semi-automatic slitter, such as disclosed in U.S. Pat. No. 4,887,502, uses a single movable stop along each arbor to position the knives for that arbor in the correct position on it. Each knife is moved against the stop and then secured on its arbor, whereupon the stop moves to the position for the next knife. In short, the knives are positioned one at a time. Even though a computer may control the movement and positioning of the stops, set up still requires time in which the slitting machine might otherwise be operating. This set up is also, to a measure, subject to operator skills.
The knives, which are essentially disks with narrow cylindrical surfaces defining their peripheries, from time to time require sharpening. This necessitates removing the knives from their respective arbors. Typically, the bearings that support the arbors at one end of the machine are backed away from the ends of the arbors and the knives are withdrawn axially from the arbors. This requires removing hubs that support the knives on the arbors and perhaps related appliances, such as stripper fingers that prevent the cut strips from acquiring curvature.
In this regard, as a cut strip emerges from the nip between the upper and lower knives of any pair, the upper knife deflects the cut strip against which it bears downwardly, so that it tends to follow the curvature of the lower knife, while the lower knife deflects the cut strip against which it bears upwardly so that it tends to follow the curvature of the upper knife. Hence, knives are organized such that along any cut strip, the knives that produce it correspond so as to deflect the strip at its sides in the same direction. While stripper fingers may exist along the sides of the knives to prevent the knives from deflecting the cut strips excessively, the strips still deflect. Indeed, each knife carries the edge of the strip along with it through a slight arc immediately after the nip at which the cut is made, and then the strip snaps back into a more planar orientation under the force exerted by the stripper finger along it and the pull of the segment of strip ahead of it. This may leave the strip with a rippled and burred edge.
Apart from that, the metal sheet upon being slit by a pair of knives tends to urge the knives apart axially. The means by which the knives are supported along their respective arbors in many slitting machines do not have the capacity to adequately resist the separating force, and as a consequence the knives deflect out of the positions best suited for slitting. Under such circumstances the cut formed by the knives may become somewhat jagged or burred.